This invention relates to an optical coupling unit applied to a bidirectional optical broadcast network using optical fibers.
By convention, the optical broadcast network is constructed by using the optical star coupler, as described in, for example, the catalogue (FIGS. 1 and 2) on Codent (trademark) issued by CODENOLL Technology Corporation, in September, 1982. This type of network is interconnected by extending one of the input side fibers of the optical star coupler and one of the output side fibers to each node. In this optical broadcast network, data from each node is transmitted through the optical star coupler to all other nodes. Each node can receive all of the data transmitted from all of the other nodes. Thus, this network is equivalent to a broadcast network (called an electrical broadcast network) using a coaxial cable called a serial bus. The optical broadcast network, unlike the electrical broadcast network, however, is unable to transmit data bidirectionally. To obtain bidirectional data transmission, the conventional optical broadcast network using the optical star coupler requires a number of optical fibers. One of the marketed electrical broadcast networks is ETHERNET (trademark of Xerox Co.).
The optical star coupler is fabricated by putting several optical fibers in a group. This group of fibers is heated, stretched, twisted and fused together, forming an optical mixer. This configuration divides the fibers into two groups. For fabricating the optical star coupler, an extremely elaborate technique is required, as is well known. Therefore, it is very difficult to fabricate an optical star coupler having many input/output ports (the number of branches). In this respect, it is very difficult to realize a large-scale optical broadcast network. Furthermore, in this type of network, the optical star coupler must be installed at a fixed location. This fixed installing location and the limitations on the number of branches make it difficult to modify and expand the optical system.